This invention relates generally to the art of assynchronous ink jet printing, and more particularly to techniques for optimizing the driving droplet ejecting pulses.
There are many specific types of printhead structures which operate to eject on demand (assynchronously) a drop of ink with a sufficient velocity to travel in a straight trajectory to a writing medium. Such a printhead typically includes an enclosed chamber whose volume is suddenly reduced to eject a droplet of ink out through a nozzle. In some applications, a single such chamber is utilized by movement with respect to a writing medium in order to generate various types of patterns and characters. A printhead having a plurality of such independently controlled printing channels is most commonly used for writing wherein the array of nozzles is swept across a writing medium line by line with the appropriate number of channels fired at each location across the line to form the desired characters.
One specific multichamber printhead is described in U.S. Pat. Nos. 4,189,734--Kyser, et al, (1980), principally FIGS. 5 and 6, 4074,284--Dexter, et al. (1978), principally FIGS. 2-4 and in co-pending application Ser. No. 058,125, filed July 16, 1979. Each channel of these printheads includes a thin piezoelectric crystal bonded to a flexible cover of the ink chamber. When an appropriate electrical potential is applied to the crystal, the cover plate is deflected downward into a chamber to supply energy to eject a droplet from that channel's nozzle.
The typical voltage pulse applied to the deflecting crystal in such devices is a square wave whose leading edge provides the drop ejecting velocity. A natural resonance results in the pressure within the ink chamber oscillating by initially going to a negative value after the droplet ejecting positive pressure pulse. The negative pressure pulse causes ink driven through said nozzle during the preceding positive pressure pulse to be pulled back into the chamber, thereby breaking free a clean droplet of ink that has been impelled toward the writing medium. The pulse is terminated either at an instant to aid in generating the negative ink pressure to break off the ink droplet in this manner, or at an instant where it would be most beneficial to counteract the natural resonance of the system and bring it to rest as quickly as possible so that it will be ready to eject a subsequent droplet. The natural resonant frequency of the system is a limitation on the maximum rate of droplets that can be emitted.
In such prior art systems, the velocity of an ejected drop can be chosen at will by varying the energy content of the drive pulse or by changing the slope of its leading edge, but the volume of the drop, and thus the size of the resulting dot, cannot be controlled independently of the velocity.
Therefore, it is an object of the present invention to drive such a device with a volume reducing pulse wave form that brings it to rest in less time than current techniques and thus increasing the rate of drops that can be emitted but without sacrificing drop volume or velocity.
It is another object of the present invention to operate such devices to emit droplets of a desired volume with an increased velocity, thereby forming characters of higher quality and permiting the printhead to be positioned a further distance from the writing medium.
It is a further object of the present invention to provide independent control of the velocity and volume of the drop.
It is also an object of the present invention to provide an improved technique of driving such a printhead device that avoids cavitation.